A concern has been raised in the IPv6 community about the privacy and anonymity of an IP level session. In order for IP level connectivity to work, the connection end points need to be unequivocally identified by unique IP addresses that reflect the topological location of the end points on the Internet. The route aggregation and network hierarchy, and therefore routing efficiency, mandate that addresses must be arranged so that subnetworks share common prefixes (typically 64 bits) and interfaces of hosts or nodes are differentiated with a suffix part, also called Interface Identifier (IID). The IPv6 autoconfiguration process specifies how such addresses can be constructed in a stateless manner. In this process, routers advertise their prefixes on a subnet, or link. The hosts append the advertised prefix with a suffix that is derived, for instance, from 48-bit IEEE 802 MAC address. This MAC address is intended to be unique. This implies that in theory one could trace the whereabouts of a host, just by monitoring the IID, thus creating a privacy concern.
This has been a motivation for RFC 3041 that complements the IPv6 stateless address autoconfiguration with a method where a node gains in addition to embedded IEEE identifier, a random interface identifier that changes over the time. A random and changing interface identifier makes eavesdropping and tracing of identity difficult and perhaps sufficiently addresses the privacy problem.
If a node wishes to be reachable for other nodes, it needs to publish its identity (name) and IP address through DNS. Dynamically changing addresses as suggested in RFC 3041 without updating the respective DNS entries would obsolete the DNS entries and the node would be isolated from its peers. If the node updates the DNS entries dynamically, the attacker or data collector could easily correlate the node's identity with the randomly varying suffix part that is required to be in effect at least during the session by a straightforward DNS lookup or with an inverse lookup. The attacker can use this kind of a DNS lookup as a vehicle for such tracking.
Many services require inverse DNS lookups for weak authentication. Therefore changing the DNS entry during an active session would exclude these services. As can be seen, the requirement for privacy and anonymity comes with a price of generating periodically new interface identifiers, added house keeping and exclusion of some services.